If you want to destination the Linux-based NAS, it would be displayed "available space is not enough".
Therefore it can not proceed with the backup wizard.
In Windows-based NAS, this symptom does not occur.
This is a known bug in the available space calculation.
Please set the backup destination to CIFS mount.
# mount -t cifs //xxxx.xxx.xxx.xxx/<shared folder> /mnt -o username=<share user>,password=<share password>
-AIP Linux 4.5.x
-AIP Linux 2016